1. Field of the Invention (Technical Field)
The present invention relates to the detection of flaws in manufactured articles which are non-homogenous.
2. Background Art
Ceramics, composites, textiles, and other nonmetallic engineered materials are superior to traditional materials in many applications with demanding performance specifications. For example, the properties of some nonmetallic materials can be varied to produce anisotropic component structural properties which improve strength to weight ratios. The use of these new materials allows the composition of a component, in addition to its physical geometry, to be spatially altered to perform to the given specifications.
However, the additional performance potential of these engineered materials greatly increases the complexity of component design, manufacture, and use. It is widely recognized that no single nondestructive evaluation (NDE) technique is capable of fully evaluating component performance. Shadwell, P. W. and Daniels, D. J., "Critical Survey of Non-Destructive Testing Techniques for Non-Conducting Materials," ERA Technology Limited Report 92-0109R (May 1992). Accordingly, a wide variety of NDE techniques have been developed to permit the verification of design models, the identification of manufacturing flaws, and the assessment of service-related damage. Many of these techniques rely fundamentally on forming spatial maps of a particular material property, such as ultrasonic propagation velocities (C-scans). The measurement of the spatial variation of a material property allows the coupling between composition, geometry, and performance to be evaluated.
The present invention is of an NDE technique and apparatus that measures geometry and structural variations in nonmetallic engineered materials via impedance mapping. Unlike previous impedance-based experiments, such as those described in Kranbuehl, D. E., "In-situ on-line measurement of composite cure with frequency dependent electromagnetic sensors," Plastics, Rubber, and Composites Processing and Applications 16:213-219 (1991), the present invention measures features of the final, cured component and not of the material properties of the bulk material. Spatial variations in the impedances measured in this technique reflect differences in geometry and structure such as thickness, arrangement of reinforcing plies, and inhomogeneities such as voids and delaminations. This technique will be applicable in a variety of manufacturing operations which form a material into a component, rather than material property analyses that focus on the material itself.
Previous work has determined that the complex permittivity (proportional to impedance) of a bulk nonmetallic material can be used to assess its chemical state. Kranbuehl, Supra. Variations in chemical bonding versus time are calculated from the complex dielectric constant using empirically determined relationships. However, these measurements are very sensitive to the geometry and physical configuration of the sample, as well as the spatial distribution of the imposed electric field. For these reasons, large, uniform samples of the nonmetallic material are required, and only gross material properties may be determined.
The applicant has previously developed the capability to create time-varying, spatially restricted, airborne electric fields through appropriate design of sensing element electrodes. Novak, J. L., and Wiczer, J. J., "A high resolution capacitive imaging sensor for manufacturing applications," Proc. IEEE Int. Conf. Robotics and Automation (1991); and U.S. Pat. No. 5,281,921, entitled "Non-contact capacitance based image sensing method and system," to Novak et al., issued Jan. 24, 1994. However, that device employs a capacitive sensing technique, which may be used only on highly conductive materials. It cannot be used on materials of arbitrary composition, as can the present invention. In addition, the capacitive technique senses the standpoint distance between the sensor and the material being tested, rendering it insensitive to the internal structure of the material.
Some work involving nondestructive testing of dielectric and conductive materials has been done using multielectrode capacitances for nondestructive evaluation. P. J. Shull et al., "Applications of capacitive array sensors to nondestructive evaluation," Rev. Prog. Quantitative Nondestructive Evaluation 7A:517-523 (1988). However, these techniques focus on a noncontacting method approaching the surface under test itself. In addition, they incorporate multiple electrodes to produce a single electric field of different shapes. The sensor of the present invention does not require access to the surface under test, but functions best when contacting the opposite side of the dielectric substrate. This invention uses multiple electrodes to generate independent electric fields to produce independent effective sensing volumes (ESVs).
A technique to measure paint thickness over a conductive coating on a dielectric substrate by contacting the dry surface itself is disclosed in U.S. Pat. No. 5,093,626, entitled "Contact measuring device for determining the dry film thickness of a paint on a conductive primer adhered to a plastic substrate," to Baer, et al., issued Mar. 3, 1992. This technique allows precise measurement of a thin, dry paint film over a conductive material on a dielectric substrate. However, it requires both contact with the surface under test and the presence of a conductive layer just below the layer of dielectric material.